The present invention relates to an electrostatographic printer, and more particularly, to an improved process control for controlling gray image level densities in an electrostatograpghic printer.
Electrostatographic printers typically employ a laser such as a laser diode as the vehicle for exposing the printer's photoreceptor, the laser being modulated in accordance with an image signal input as the beam is scanned across the photoreceptor line by line while the photoreceptor is moving in a direction perpendicular to the direction in which the beam is scanned. The dot-like image pattern created, which forms a duplicate of the image represented by the image signals, is thereafter developed, transferred to a substrate such as paper, and fixed as by fusing to provide a permanent copy or print.
The quality of the image produced depends on a number of factors, one important factor being resolution, i.e., the number of dots or spots laid down per unit of distance. Essentially, the higher the resolution, the more acceptable the image produced. And while only black and white image dots may be used, the image quality is improved by the use of one or more intermediate gray levels. In applications where this is done, the gray level or levels may be controlled by controlling the on-time of the laser. For example, in a write `white` system, by maintaining the beam on longer, a larger portion of the image dot is exposed. This results in development of a lighter gray spot since the amount of toner is spread over a larger area. Similarly, where the beam on-time is curtailed, the area of the image spot exposed is less, leading to a higher concentration of toner in a smaller area, giving the impression of a darker spot. However, a problem with using one or more intermediate gray levels to improve image quality is the stability of the gray level output.
Where the printer employs an IR laser diode and IR sensitive photoreceptor, the development system used in that case may be a relatively simple single component type development system. In that type of development system, toner attraction is electro-voltic. Further, a high level of developer isolation is possible, the relationship between developer mass and the developer biasing current is optimized, and relatively high ac-to-dc current ratios are possible. And in view of the linear relationship that exists between DMA and developer current, and the high degree of developer isolation, in theory the dc component of the combined ac/dc developer bias should be usable to accurately monitor developer output density and control exposure so as to maintain a substantially constant gray level performance. Unfortunately, accurately isolating the dc component of the combined ac/dc developer bias has proved to be a problem.